Housingless load cell

ABSTRACT

The invention relates to a non-encased weighing cell which with a view to resistance to environmental influences and high measuring accuracy has been improved to the effect that at least the predominant part of the surface ( 12 ) and/or of the surface ( 14 ) is inclined such that in the installed state, as intended, of the weighing cell any water hitting the weighing-cell body ( 1 ) can flow away, wherein the angle α 1  between the inclined part of the surface ( 12 ) and the perpendicular circumferential surface of the trunk ( 3 ) and/or the angle β 2  between the inclined part of the surface ( 14 ) and the circumferential surface of the foot ( 4 ) is larger than 90°, and in particular is between 93° and 120°. Furthermore, a non-encased weighing cell is stated in which the balancing electronics are provided in one of the transverse holes ( 7 ). Finally, a weighing-cell is improved to the effect that the circumferential surface of the foot ( 4 ) comprises at least one groove ( 24 ) that is essentially perpendicular, which groove together with a projecting element that is provided on the base plate provides a device to prevent torsion.

The invention relates to a non-encased weighing cell with aweighing-cell body, comprising a head, a trunk and a foot, wherein thehead is set apart from the trunk by at least one top recess, whichextends in a rotationally symmetrical manner around the longitudinalaxis of the weighing-cell body, wherein the recess comprises a surfacethat tapers off towards the trunk and a surface that tapers off towardsthe head; the trunk is essentially cylindrical in shape, and the trunkis set apart from the foot by at least one bottom recess that extends ina rotationally symmetrical manner around the longitudinal axis of theweighing-cell body, wherein the recess comprises a surface that tapersoff towards the foot and a surface that tapers off towards the trunk.

As a rule, weighing cells, for example pendulum weighing cells for usein weighing machines for road vehicles, comprise a measuring element towhich a wire strain gauge has been glued, which measuring elementhereinafter is referred to as a weighing-cell body.

The weighing-cell body is made of a high-strength metal material, as arule of steel; it comprises a head, a foot, and a trunk arranged inbetween. In a suitable position on the surface of the weighing-cellbody, usually on the essentially cylindrical trunk, wire strain gaugesare arranged. These wire strain gauges, as a rule Konstantan or Karmawire strain gauges, are usually electrically wired as a Wheatstone fullbridge.

To protect the wire strain gauges from water, humidity or dirt, aseparate housing is arranged around the weighing-cell body. Furthermore,a balancing chamber has been affixed to this housing, which balancingchamber comprises electronics for electrically balancing the weighingcell, for example for determining the zero point, the characteristicvalue, the temperature behaviour etc., and if necessary a signalprocessing device. This balancing chamber is also used for protectingthe electronic components from humidity and dirt.

Furthermore, weighing cells are known in which in the trunk, on an axisperpendicular to the longitudinal axis of the weighing-cell body, atransverse through-hole is provided, in whose centre a membrane hassubsequently been installed, onto which membrane the wire strain gaugesare glued. Instead of providing one through-hole and a separatemembrane, it is also known to provide two transverse holes that arepositioned on the same axis, wherein the depth of said transverse holesis less than the radius of the trunk, and wherein said transverse holesform a web between themselves, which web assumes the function of themembrane, and onto which web the wire strain gauges are glued.

In order to achieve a targeted load introduction from the end surfacesof the head and of the foot into the trunk, in particular in thedirection of a web or of a membrane, recesses that are aligned so as tobe rotationally around symmetrical the longitudinal axis of theweighing-cell body are provided, through which recesses the head is setapart from the trunk, and the trunk is set apart from the foot.

Actually, in those cases where the wire strain gauges are arranged on aweb in the interior of the trunk said wire strain gauges are betterprotected when compared to those cases where they are arranged on theperpendicular circumferential surface of the trunk so that such aweighing cell can be used even without a separate housing that enclosesthe entire weighing cell, which results in a considerable costreduction. However, it is still possible for humidity and dirt tocollect in various positions on the weighing-cell body, including in thetransverse holes, and in this way damage the weighing-cell body, forexample through corrosion. In particular, in the region of therotationally symmetrical recesses that set the trunk apart from the heador the foot, moisture can collect.

Furthermore, even if no separate housing that encloses the entireweighing cell is provided, a balancing chamber to accommodate thebalancing electronics is still required.

The known weighing cell is associated with a further disadvantage inthat it can be subject to torsion during operation, which results inmeasuring errors.

Starting with the problems shown above, it is the object of the presentinvention to create a non-encased weighing cell which is particularlyresistant to environmental influences, and which features very goodmeasuring accuracy.

According to the invention the above-derived and shown object is firstof all met in that in a non-encased weighing cell with thecharacteristics of the precharacterising part of claim 1 at least thepredominant part of the surface of the top recess, which surface tapersoff towards the trunk, and/or the predominant part of the surface of thebottom recess, which surface tapers off towards the foot, is inclinedsuch that in the installed state, as intended, of the weighing cell anywater hitting the weighing-cell body can flow away, wherein the angle α₁between the inclined part of the surface of the top recess, whichsurface tapers off towards the trunk, and the perpendicularcircumferential surface of the trunk and/or the angle β₂ between theinclined part of the surface of the bottom recess, which surface tapersoff towards the foot, and the circumferential surface of the foot islarger than 90°, and in particular is between 93° and 120°. A value of117.5° has been shown to be particularly suitable.

In this way a situation can be achieved in which in the region of therecesses almost no plane surfaces perpendicular to the longitudinal axisof the weighing-cell body are present any longer, on which surfacesmoisture and dirt can accumulate. It is quite possible for a narrowannular region in the end region of the surface tapering off towards thetrunk or towards the head to be present, which annular region is notinclined but extends so as to be perpendicular in relation to thelongitudinal axis of the weighing-cell body, which annular region,however, due to its small size when compared to the much larger inclinedpart of the surface of the respective recess does not have any effect onthe outflow of the water. Thus, in longitudinal section along thelongitudinal axis of the weighing-cell body, the surface line of the toprecess and/or of the bottom recess in the respective regions taperingoff downwards always have an incline which ideally extends right to theedge of the trunk or of the foot. Water reaching this region, ormoisture collecting in this region drains in this way, wherein dirtparticles are also flushed away. Thus the danger of corrosion in theregion of the recesses is clearly reduced.

In addition it can be provided for at least the predominant part of thesurface of the top recess, which surface tapers off towards the head,and/or the predominant part of the surface of the bottom recess, whichsurface tapers off towards the trunk, is inclined, such that in theinstalled state, as intended, of the weighing cell any water hitting theweighing-cell body can flow away, wherein the angle α₂ between thesurface of the bottom recess, which surface tapers off towards thetrunk, and the perpendicular circumferential surface of the trunk and/orthe angle β₁ between the surface of the top recess, which surface tapersoff towards the head, and the circumferential surface of the head islarger than 90°, and in particular is between 93° and 120°, wherein inthe upper region of the recesses, too, an inclined surface is formedfrom which surface water can drain off more easily than from ahorizontal surface. A value of 117.5° has been shown to be particularlysuitable.

If the diameter of the head and/or the diameter of the foot is smallerthan the diameter of the trunk, the surface of the top recess, whichsurface tapers off towards the trunk, can be inclined also in thatregion that is not covered by the head, and/or the surface of the bottomrecess, which surface tapers off towards the trunk, can be inclined alsoin that region that is not covered by the foot. The designation“covered” refers to the region of the inclined surfaces, which region ina top or bottom view of the weighing-cell body is covered by the head orthe foot and is thus not visible.

Advantageously, the longitudinal section of the top recess and/or of thebottom recess has at least in part a shape of the surface line that iscurved, in particular shaped in a circular arc, an ellipsis or aparabola. However, other shapes of the surface line are imaginable,provided the gradient of the surfaces in the region of the recess issufficient for the moisture to drain off optimally.

In a further advantageous embodiment the transition region between thesurface of the top recess, which surface tapers off towards the trunk,and the perpendicular circumferential surface of the trunk and/or thetransition region between the surface of the bottom recess, whichsurface tapers off towards the foot, and the circumferential surface ofthe foot, is inclined or rounded. Accordingly, it is imaginable that ineach case the transition region between the surface of the top recessthat tapers off towards the head and the circumferential surface of thehead is inclined or rounded. In this way edges are prevented and insteada shape is created from which water can drain off well.

Also in the transition region between the interior surface of thetransverse holes and the perpendicular circumferential surface of thetrunk, edges can be avoided by using inclined or rounded shapes.

According to the invention, the object derived and shown above is met inthat in a non-encased weighing cell with the characteristics of theprecharacterising part of claim 11 wire strain gauges are arranged onthe web, which wire strain gauges are electrically connected tobalancing electronics that are arranged in one of the transverse holes.In this way there is no longer a need to provide a separate balancingchamber to accommodate the balancing electronics, which balancingelectronics can comprise a printed circuit board which on one side cancomprise a device for temperature compensation, in particular ameandering layer of nickel. In this way the balancing electronics arearranged directly in the region of the wire strain gauges.

Advantageously the wire strain gauges are enclosed by a castingcompound, for example a casting compound made of plastic, in particularflexible plastic, so that said wire strain gauges are now fullyprotected from humidity or dirt in the transverse holes. At the sametime the casting compound can serve as an attachment for the printedcircuit board in that said printed circuit board is at least partiallyembedded in the casting compound.

A particularly good measuring result is achieved if on each side of theweb at least one wire strain gauge is arranged. In this case, toestablish an electrical connection between the wire strain gauges andthe balancing electronics a borehole can be provided in the web, throughwhich web the connecting lines lead. Furthermore, the weighing-cell bodycan comprise a cable bushing through which the signal lines and currentsupply lines lead from one of the transverse holes to the outside into aweighing-cell cable. Advantageously, this cable bushing is sealed offagainst humidity and dirt.

Particularly good protection from humidity and dirt is achieved if thetransverse holes are covered by covers. Sealing off the covers can takeplace by means of welding, in particular microplasm welding. In this wayoptimal sealing action of the covers is achieved. It is also possible,as an alternative or in addition, to glue, screw and/or tighten thecovers.

Finally, according to the invention, the previously derived and shownobject is met in that in a weighing cell with the characteristics of theprecharacterising part of claim 23 the circumferential surface of thefoot comprises at least one groove that is essentially perpendicular,which groove in each instance can interact, in the installed state ofthe weighing cell, with an element that is provided on the base plateand that projects therefrom. Such a projecting element can for examplebe a pin, which in particular extends perpendicularly to thelongitudinal axis of the weighing-cell body. Also imaginable are severalgrooves that are circumferentially arranged at even spacing, as well asseveral corresponding elements in the base plate. In this way a deviceto prevent torsion of the weighing cell can be created, which deviceensures permanent positioning of said weighing cell, thus clearlyreducing the risk of measuring errors occurring.

Below, the invention is explained with reference to drawings that showexemplary embodiments. It is shown schematically in:

FIGS. 1 a) and b) the weighing cell according to the invention for twodifferent load stages;

FIG. 2 a longitudinal section of the weighing cell shown in FIG. 1 a);

FIG. 3 a cross section of the weighing cell shown in FIG. 1 a); and

FIGS. 4 a) and b) a longitudinal section of two exemplary embodiments ofthe weighing cell according to the invention.

FIGS. 1 a) and b) show a non-encased weighing cell for two differentload stages, namely in FIG. 1 a for a load stage of 50 t and in FIG. 1 bfor a load stage of 25 t. In each instance the weighing cell showncomprises a weighing-cell body 1, which in turn comprises a head 2, atrunk 3 and a foot 4. The head 2 is set apart from the trunk 3 by a toprecess 5, which extends in a rotationally symmetrical manner around thelongitudinal axis of the weighing-cell body 1. Towards the bottom thetrunk 3 is set apart from the foot 4 by a corresponding recess 6. In itsperpendicular circumferential surface the essentially cylindrical trunk3 comprises two transverse holes 7 and 8, of which only one transversehole 7 is shown in the view selected in FIGS. 1 a) and b). In each casethe transverse holes are tightly sealed off by a welded-on cover 9.

Furthermore, a cable bushing 10 is provided in the weighing-cell body 1,through which cable bushing 10 the signal lines and current supply linesare led from the front transverse hole 7 towards the outside in aweighing-cell cable 11.

Since the weighing cell depicted does not comprise a separate casing andis thus directly exposed to environmental influences, in particular towater, humidity, dirt etc., there are no flat surfaces extendingperpendicularly to the longitudinal axis. It is clearly illustrated thatthe surface 12 that tapers off towards the trunk 3, and the surface 13of the top recess 5, which surface 13 tapers off towards the head 2, areinclined so that water or moisture drains off directly and cannotcollect. The surface 14 that tapers off towards the foot 4, and thesurface 15 that tapers off towards the trunk 3, of the bottom recess 6,are designed correspondingly. Similarly, the transition region 16between the internal surface of the transverse holes 7 and 8 and of theperpendicular circumferential surface of the trunk 3 is somewhatinclined so that here, too, water can drain off directly.

The principle on which the non-encased weighing cell according to theinvention is based is further illustrated in FIG. 2. FIG. 2 shows alongitudinal section of the weighing cell from FIG. 1 a). Here, too, theillustration shows that there is no horizontal surface that extendsperpendicularly to the longitudinal axis, on which surface the humiditycould collect. Thus the two recesses 5 and 6 are designed such that theangle α₁ between the surface 12 of the top recess 5, which surfacetapers off towards the trunk 3, and the perpendicular circumferentialsurface of the trunk 3 exceeds 90°. The same applies to the angle β₂between the surface 14 of the bottom recess 6, which surface tapers offtowards the foot 4, and the circumferential surface of the foot 4.Finally, both the angle α₂ between the surface 15 of the bottom recess6, which tapers off towards the trunk 3, and the perpendicularcircumferential surface of the trunk 3, as well as the angle β₁ betweenthe surface 13 of the top recess 5, which surface tapers off towards thehead 2, and the circumferential surface of the head 2 are larger than90°. In the region of the recesses 5 and 6 the surface line isapproximately parabolic.

FIG. 2 further shows the two transverse holes 7 and 8, which aresufficiently deep for a web 17 to be formed between them, at both sidesof which web 17 a wire strain gauge 18 has been glued on. In eachinstance the wire strain gauge 18 is enclosed by a casting compound madeof flexible plastic. The relatively soft casting compound 19 ensuresthat the wire strain gauges 18 are permanently protected againstmechanical and climatic influences, in particular also during thebalancing procedure.

In each instance a separate cover 20, which has been welded on, sealsthe two transverse holes 7 and 8 off towards the outside.

Furthermore, the transverse hole 7 comprises the balancing electronicscomprising a printed circuit board 21 and a meandering layer 22, made ofnickel, for temperature compensation. The balancing electronics areaffixed in that one face of the printed circuit board 21 is embedded inthe casting compound 19. In this way there is no need whatsoever for aseparate balancing chamber since the latter is formed by the transversehole 7.

FIG. 2 further shows a perpendicular groove 24 with which a pin thatextends perpendicularly to the longitudinal axis of the weighing cellcan interact, wherein said pin is firmly connected to a base plate whichin the installed state carries the weighing cell.

This device to prevent torsion ensures permanent positioning to reducethe sensitivity error when the weighing cell is at an inclined position.Of course it is also possible to provide several grooves, for examplethree grooves, which advantageously are distributed on the circumferenceof the weighing-cell body so as to be regularly spaced apart.

FIG. 3 shows a cross section of the weighing-cell from FIG. 1 a). Theillustration clearly shows that a borehole 23 extends through the web 17that is in place between the transverse holes 7 and 8, through whichborehole 23 the lines for electrical connection of the wire straingauges 18 to the balancing electronics lead. Furthermore, the cablebushing 10 is shown, through which the signal lines and current supplylines lead from the transverse hole 7 towards the outside into theweighing cell cable 11.

FIGS. 4 a) and b) show a longitudinal section of two embodiments of theweighing cell according to the invention. FIG. 4 a) shows a weighingcell in which the predominant part of the surface 12 of the top recess5, which surface tapers off towards the trunk 3, as well as thepredominant part of the surface 15 of the bottom recess 6, which surfacetapers off towards the trunk 3, is inclined, and that only in the endregion of the surfaces 12 and 15 is there a narrow annular region whichextends perpendicularly to the longitudinal axis of the weighing-cellbody 1.

FIG. 4 b shows an embodiment according to the solution according to theinvention, which embodiment is to be preferred over that shown in FIG. 4a, in which preferred embodiment the surfaces 12 and 15 are inclined upto the upper or lower edge of the trunk 3. FIGS. 4 a) and b) furthershow that the surface 12 and the surface 15 are inclined also in thatregion that is not covered by the head 2 or by the foot 4. In eachinstance the covered region is to the right of the dashed line, whilethe non-covered region is to the left of the dashed line.

1. A non-encased weighing cell with a weighing-cell body (1), comprisinga head (2), a trunk (3) and a foot (4), wherein the head (2) is setapart from the trunk (3) by at least one top recess (5), which extendsin a rotationally symmetrical manner around the longitudinal axis of theweighing-cell body (1), wherein the top recess (5) comprises a firstsurface (12) that tapers off towards the trunk (3) and a second surface(13) that tapers off towards the head (2); the trunk (3) is essentiallycylindrical in shape, and the trunk (3) is set apart from the foot (4)by at least one bottom recess (6) that extends in a rotationallysymmetrical manner around the longitudinal axis of the weighing-cellbody (1), wherein the bottom recess (6) comprises a third surface (14)that tapers off towards the foot (4) and a fourth surface (15) thattapers off towards the trunk (3), wherein at least the predominant partof at least one of the first surface (12) and the third surface (14) isinclined such that in the installed state, as intended, of the weighingcell any water hitting the weighing-cell body (1) can flow away, whereinat least one of an angle α₁ between the inclined part of the firstsurface (12) and the perpendicular circumferential surface of the trunk(3) and an angle β₂ between the inclined part of the third surface (14)and the circumferential surface of the foot (4) is larger than 90°. 2.The non-encased weighing cell according to claim 1, wherein at least apredominant part of at least one of the second surface (13) the fourthsurface (15) is inclined such that in the installed state, as intended,of the weighing cell any water hitting the weighing-cell body (1) canflow away, wherein at least one of the angle α₂ between the fourthsurface (15) and the perpendicular circumferential surface of the trunk(3) and the angle β₁ between the second surface (13) and thecircumferential surface of the head (2) is between 93° and 120°.
 3. Thenon-encased weighing cell according to claim 1, wherein at least one ofa diameter of the head (2) and a diameter of the foot (4) is smallerthan a diameter of the trunk (3), wherein the first surface (12) isinclined also in that region that is not covered by the head (2), and/orthe fourth surface (15) is inclined also in that region that is notcovered by the foot (4).
 4. The non-encased weighing cell according toclaim 1, wherein the longitudinal section of at least one of the toprecess (5) and the bottom recess (6) has, at least in part, a shape ofthe surface line that is curved.
 5. The non-encased weighing cellaccording to claim 4, wherein the shape is selected from the groupconsisting of: circular arc, ellipsis and parabola.
 6. The non-encasedweighing cell according to claim 1, wherein at least one of a transitionregion between the first surface (12) of the top recess (5), and aperpendicular circumferential surface of the trunk (3) and a transitionregion between the third surface (14) of the bottom recess (6) and thecircumferential surface of the foot (4), is inclined or rounded.
 7. Thenon-encased weighing cell according to claim 1, wherein an upper endsurface of the head (2) is a spherical surface.
 8. The non-encasedweighing cell according to claim 1, wherein in the trunk (3) on an axisperpendicular to the longitudinal axis of the weighing-cell body (1) twotransverse holes (7, 8) with a depth less than a radius of the trunk (3)are provided, wherein said transverse holes (7, 8) form a web (17)between themselves, and wherein wire strain gauges (18) are arranged onthe web (17), which wire strain gauges (18) are electrically connectedto balancing electronics that are arranged in one of the transverseholes (7).
 9. The non-encased weighing cell according to claim 8,wherein the balancing electronics comprise a printed circuit board (21)that, on one side, comprises a device for temperature compensation. 10.The non-encased weighing cell according to claim 9, wherein the devicefor temperature compensation is a meandering layer (22) of nickel. 11.The non-encased weighing cell according to claim 8, wherein the wirestrain gauges (18) are enclosed by a casting compound (19).
 12. Thenon-encased weighing cell according to claim 11, wherein the castingcompound (19) is a flexible plastic material.
 13. The non-encasedweighing cell according to claim 11, wherein the printed circuit boardis at least partly embedded in the casting compound (19).
 14. Thenon-encased weighing cell according to claim 8, wherein at least onewire strain gauge (18) is arranged on each side of the web (17).
 15. Thenon-encased weighing cell according to claim 14, wherein, for electricalconnection between the wire strain gauges (18) and the balancingelectronics, a borehole (23) is provided in the web (17).
 16. Thenon-encased weighing cell according to claim 8, wherein theweighing-cell body (1) comprises a cable bushing (10) through which thesignal lines and current supply lines lead from one of the transverseholes (7) to the outside into a weighing-cell cable (11).
 17. Thenon-encased weighing cell according to claim 16, wherein the cablebushing (10) is sealed off from humidity and dirt.
 18. The non-encasedweighing cell according to claim 8, wherein the transverse holes (7, 8)are closed by covers (9).
 19. The non-encased weighing cell according toclaim 18, wherein the covers (9) are welded and/or glued and/or screwedand/or riveted into place.
 20. A weighing cell, in particular anon-encased weighing cell according to claim 1, with a weighing-cellbody (1), wherein the circumferential surface of the foot (4) comprisesat least one groove (24) that is essentially perpendicular, which groovein each instance can interact, in the installed state of the weighingcell, with an element that is provided on the base plate and thatprojects therefrom, for example a pin that extends in particular so asto be perpendicular in relation to the longitudinal axis of theweighing-cell body.